i need some clarification about not having global shopping cart?

let say if a customer click in one product and that particular store that sell this product open up, the customer add that product to the cart and want to exit the store to go shop at other stores, does marketpress will show a message saying to finish paying for the item first before exiting?

second, should i put an exit button so they can go back to the main store? any help will be greatly appreciated.